More than 10,000 separate species dwell naturally in the human gut, playing important roles in maintaining the health of the human digestive and immune systems. These organisms are so numerous that they outnumber our body's own cells by 10 to 1.

Scientists still know very little about the composition of human gut flora, or about how changes in this internal ecosystem can affect our health. But the research conducted thus far suggests that gut flora may play a role in obesity and certain diseases such as inflammatory bowel disease. Studies have also shown that diets high in fat and sugar may change the makeup of our internal ecosystems.